1. Field of the Invention
The present invention relates to an antilock braking apparatus and more particularly to an antilock braking apparatus adapted to operate by way of the steps of making a decision as to whether or not a tire having a smaller effective radius such as provisional tire or the like is mounted on a vehicle on the basis of difference in angular speed between arbitrarily selected wheels and then generating a release signal for releasing antilock control immediately after it is found that a tire having a smaller effective radius such as provisional tire or the like is mounted thereon, whereby a responsive function of the antilock braking apparatus is assured with respect to the existence of such a tire as having a smaller effective radius.
2. Description of the Prior Art
As is generally known, a rate of difference in speed between vehicle speed and peripheral speed of wheels to the vehicle speed, that is, a wheel slip rate tends to increase as a braking force increases during braking operation. A conventional antilock braking apparatus is usually constructed so as to inhibit application of braking force to wheels when a wheel locking is about to occur due to excessive braking force and then release inhibition of braking force when the wheels again resume the original peripheral speed and a fear disappears of causing wheel locking, wherein the aforesaid wheel slip rate functions is an essential input factor for controlling braking force. However, to allow a wheel slip rate to be effectively utilized for control operation, peripheral speed of the wheels and vehicle speed should be first detected.
In the case of standard tires being mounted on the vehicle, their effective radius has been previously known so that the peripheral speed of a wheel can be readily calculated from the number of revolutions or angular speed of the wheel if it is known. A wheel speed detector employed for the antilock braking apparatus is usually designed so as to detect angular speed of a wheel and thereafter generate signals as a wheel speed signal having a frequency in proportion to that angular speed. It is natural that a frequency of wheel speed signal is in proportion to peripheral speed of the wheel.
To provide means for detecting vehicle speed there has been already proposed various methods in which it is directly or indirectly detected. In the conventional antilock control apparatus a method of estimating vehicle speed from peripheral speed of a wheel is usually employed mainly for the economical and technical reasons.
In this method calculation of peripheral speed of a wheel is carried out by detecting angular speed of the same and vehicle speed is then estimated from the thus calculated peripheral speed on the assumption that standard tires are mounted on a vehicle. If a so-called provisional tire having a smaller effective radius than that of standard tires is fitted onto a wheel, it results that peripheral speed of the wheel is estimated higher than its actual one and thereby vehicle speed which is usually estimated by utilizing peripheral speed of the wheel as an input information is likely to be estimated excessively high.
Once vehicle speed is estimated higher than its actual one in the antilock braking apparatus, a wheel slip rate of a wheel with a standard tire mounted thereon which is calculated from the estimated vehicle speed is determined higher than its actual one. As a result braking force is excessively suppressed to cause a substantial reduction of the braking effect.
A so-called provisional tire is normally equipped on a vehicle in view of an occurance of puncture in any one of the standard tires and therefore a plurality of provisional tires are very rarely used on a plurality of wheels simultaneously.